1. Field of the Invention
The present invention relates to a method of combining or joining workpieces in which two or more workpieces are set in position on a jig having mounted thereon clamp units and then these workpieces are combined while they are clamped by the clamp units.
2. Description of the Related Art
In Japanese Published Unexamined Patent Application No. 153387/1989, the following method is conventionally known. Namely, clamp units are arranged to be movable. In the first place, workpieces are combined by clamping them together in a condition or state in which the clamp units are positioned in predetermined reference positions to be determined by design data. Then, the accuracy of a product thus assembled by this combining work is optically measured. Based on the result of this measurement, the positions of the clamp units are corrected and the combining work and its accuracy measurement are performed with the next workpieces. These steps are repeated until a product having a desired accuracy can be obtained.
An example is now considered in which a product having a closed cross section is assembled by combining two workpieces Wa, Wb as shown in FIG. 1. For example, if the workpiece Wa has a distortion .delta. as shown in FIG. 2A, the workpiece Wa will be forcibly deflected by the amount of the distortion .delta. as shown in FIG. 2B at the time of clamping. If the product W is assembled by combining the workpieces in this state, the product W will give rise to a distortion .delta.' by springing back when it is unclamped as shown in FIG. 2C.
In the above-described conventional art, this kind of distortion .delta.' in the product is measured, and the positions of the clamp units are corrected in a direction to decrease the distortion, followed by the combining work of the next workpieces. In this kind of feedback system of correction, however, it is impossible to obtain a product of higher accuracy from the very beginning, with the result that much waste occurs. Further, workpieces have an inherent dispersion of their own within the same lot in addition to a dispersion from lot to lot. In case the distortion of the workpieces of this time is different from the distortion of the workpieces of the last time, the correction based on the measured data of the product obtained by the combining the workpieces of the last time does not apply to the workpieces of this time any more. It follows that the conventional feedback system of correction cannot deal with the dispersions of the respective workpieces.